Patent GB-1234388-A (GB)
IPC: F16H61/02
Filed: 1967-10-14
Abstract: 1,234,388. Change speed control. AUTO TRANSMISSIONS Ltd. 4 Oct., 1968 [14 Oct., 1967], No. 46935/67. Heading F2D. A fluid control circuit for operating a ratio selecting friction engaging element in a changespeed gearing includes means to regulate the fluid pressure between high and low values at which one or the other ratio is engaged, the regulating means being arranged to reduce the rate at which the pressure falls from the high to the low value for part of the time that the friction engaging element is dsiengaging one ratio and engaging the other ratio before dropping at a greater rate to the low value whereby the shock of engagement of the other drive ratio is reduced. A lubricating oil pump 10 discharges via duct 12, filter 13 and a duct 15 to a combined accumulator and pressure relief valve 42 and via a duct 14 to a control valve 24 and a cylinder 16 having a piston 18. The latter moves a clutch element 20 of an epicyclic gearing against spring 21 to engage one ratio (1st ratio). When the fluid pressure is reduced the springs 21 move element 20 to engage the other ratio. (2nd ratio). A spring may be fitted in space 23 to cushion the engagement of the ratios or piston 18 may be omitted. Operation.-When a control valve operating solenoid 41 is de-energized and pump 10 is inoperative the circuit is in the Fig. 1 position. 1st to 2nd ratio change.-Pump 10 pressurizes a chamber 70 to move a relief piston 69 to the left compressing a spring 71 until passages 75 are uncovered to pass oil via duct 76 to lubricate the epicyclic gearing at a pressure determined by a relief valve 77. When the solenoid 41 is energized a valve spool 34 engages seating 33 to disconnect duct 39 from an exhaust restrictor 40 and connect it to duct 38, the low pressure now acting on an accumulator piston 53. This moves to the right against the force of the spring system causing the relief piston 69 to restrict the flow through passages 75 thereby raising the system pressure which accelerates the movement of the accumulator piston 53 giving a further pressure rise. This continues until spring 56 is compressed until the cup member 58 abuts the accumulator piston, spring 71 is compressed until flange 72 abuts the cup member 58 and spring 64 is also compressed by the cup member 58 engaging a flange 65 of a slidable sleeve 63. The system pressure becomes stabilized at its high value according to the position assumed by relief piston 69 balanced by spring 71. The rising pressure acting on piston 18 produces a relatively quick change of ratio, any shock of engagement being prevented by the restricted rate of pressure rise due to the filling of the accumulator. 2nd to 1st ratio change.-De-energization of solenoid 41 permits spring-return of valve 34 into engagement with seating 32. The pressure on accumulator piston 53 starts to drop as fluid is exhausted through restrictor 40. The springs move the accumulator piston 53 to the left arid this results in a corresponding leftwards movement of the relief piston 69 so that pressure in the system including that acting on piston 18, is proportional to that acting on the accumulator piston 53. Initially only spring 71 is extending causing a steep pressure fall with small fluid expulsion, clutch element 20 still being engaged but with some slip so that the application of power causes rise of engine speed with some drive. When pin head 74 engages cup (67) (Fig. 3, not shown) the combined forces of springs 56 and 64 balance the pressure acting on the accumulator piston 53 and further pressure fall allows the extension of these springs to expel fluid through the restrictor 40 which maintains a slow rate of pressure fall until the shoulder 62 abuts the sleeve 63, Fig. 4 (not shown). The preload of spring 64 causes a steep pressure drop until springs 21 overcome the fluid pressure on piston 18 to cause engagement of the other ratio. A further period of slow pressure drop is caused by the extension of spring 56 until the pin head 61 abuts the cup member 58 whence pressure drops steeply until flange 72 moves away from cup member 58 so that spring 66 becomes operative and the system assumes its low-pressure condition. In a further embodiment to alter the rate of pressure reduction spring 64 and sleeve 63 are omitted and pin 59 is fastened to the accumulator piston 53, Fig. 7 (not shown). In a further modification of this embodiment the spring (66) is replaced by a spring (80), Fig. 8 (not shown), between parts corresponding to cup member 58 and washer 73 of Fig. 1. In all embodiments the springs may be replaced by pneumatic or hydraulic elements. Restrictor 40 may be arranged in duct 39 to obtain a self-clearing effect due to flow reversal.